Filling devices of this kind are used in particular in the dispensing of small dosage amounts as required for example in the field of pharmaceuticals. The target containers are often set on a balance in order to weigh the amount of substance delivered by the dosage-dispensing device, so that the substance can subsequently be further processed in accordance with given instructions.
The substance to be dispensed is held for example in a source container or reservoir which is equipped with a dispensing head. It is desirable if the substance to be dispensed is delivered to the outside through an opening of the dosage-dispensing device, so that at the end of the filling process a predetermined target mass of substance will have been received by the target container. It is of importance here that the actual amount of mass in the target container agrees as accurately as possible with the predetermined target mass. It is further important that the filling process can be performed as rapidly as possible.
Known from the existing state of the art are dosage-dispensing methods which are based on a volumetric measurement of the amount of substance delivered. For a substance of density ρ, a valve with a variable aperture cross-section A and, associated with these parameters, a resultant outflow velocity u of the substance, the mass mz of the substance in the target container is obtained from the equation
                              m          z                =                ⁢                              ∫                          t              open                                      t              close                                ⁢                                                    m                .                            ⁡                              (                t                )                                      ⁢                          ⅆ              t                                                              =                ⁢                              ∫                          t              open                                      t              close                                ⁢                      ρ            ⁢                                                  ⁢                                          V                .                            ⁡                              (                t                )                                      ⁢                          ⅆ              t                                                              =                ⁢                              ∫                          t              open                                      t              close                                ⁢                                    ρ              ⁡                              (                Au                )                                      ⁢                          (              t              )                        ⁢                          ⅆ              t                                                              =                ⁢                              ∫                          t              open                                      t              close                                ⁢                      ρ            ⁢                                                  ⁢                          A              ⁡                              (                t                )                                      ⁢                          u              (                              A                ,                h                ,                d                ,                …                            ⁢                                                          )                        ⁢                          ⅆ              t                                          
In particular the outflow velocity u is subject to many factors such as for example the cross-sectional area A of the valve aperture, the hydrostatic pressure resulting from the fill height h of the substance in the reservoir, and the rheological properties of the substance such as for example the grain size d of the powder. The rheological properties are often very complex and are subject to factors which are not known with any degree of precision. It is for example difficult to take the flow retardation into account which occurs in Bingham media or powders at the beginning of the flow movement. Particularly in the filling of pulverous substances, factors such as for example grain size, moisture content, and surface properties of the individual particles, have a major influence.
In U.S. Pat. No. 4,893,262 a controller system for the filling of containers is disclosed. The system is optimized in a process that extends over several filling cycles, wherein the mass flow is optimized from one cycle to the next and the filling time is adjusted until the dispensed mass agrees as accurately as possible with the predetermined target mass. This system is used primarily for the filling of large quantities where the requirements concerning accuracy are considerably lower than for the system of the present invention. The fact that the optimization extends over several cycles represents a further problem, as the predetermined target mass is actually attained with the required accuracy only after several trial cycles. The substance dispensed during these trial cycles cannot be used anymore, since it could have been contaminated in the process of dispensing and subsequent removal from the target container. This is a decisive disadvantage especially when the substances being dispensed are expensive.
A method and apparatus to dispense a small mass of particles accurately and reproducibly are disclosed in U.S. Pat. No. 6,987,228 B1. The apparatus includes a controller unit which serves to control the amount of energy imparted to a sieve which holds the particles to be dispensed. Energizing the sieve has the effect that a small quantity of the particles in the sieve will fall on a balance that is arranged below the sieve. Based on the weight measured by the balance, the controller unit controls the amount of energy that is applied to the sieve. The amount of energy being introduced can be controlled as a function of the amount of mass that remains to be dispensed, whereby the outflow rate of the particles can be varied. This arrangement has the problem that with the use of the sieve, only substances in powder form can be dispensed. For other free-flowing substances, particularly for liquids, this method is not suitable. Even when pulverous substances are being dispensed, there are drawbacks inherent in this method, as different sieves have to be used depending on the grain size of the substance. The essential disadvantage concerns the control of the energy supplied to the sieve as a function of the weighing signal. Because of the time delay in the response of the balance, the filling process would have to be performed at a slow enough speed to allow enough time for the balance to respond. As a result, the filling process would take a very long time.
A system to control the filling of containers is disclosed in U.S. Pat. No. 4,762,252. To determine the mass flow rate during the filling process, the change in the weight of the reservoir is measured. The mass flow rate that is determined in this way is compared to a desired flow rate. If the measured flow rate deviates too strongly from the desired flow rate, the mass flow rate is adjusted accordingly. The system described in this reference is suitable for dispensing about 25 to 50 kilograms per hour. In the filling of small quantities of the sizes required in the field of pharmaceuticals, a high level of accuracy is required and small measurement inaccuracies can have a significant influence on the fill mass. At the same time, the filling process should take as little time as possible.
It is therefore an object of the present invention to provide a method and an apparatus serving to accurately and reproducibly dispense a predefined small fill quantity of a free-flowing substance and having the attributes of being simple, fast and accurate.